The prior art has disclosed hydrostatic transmissions for mobile working machines in which a hydrostatic pump (primary unit) and at least one hydrostatic motor (secondary unit) are fluidically connected to one another by means of a closed hydraulic circuit. A primary unit may be situated with multiple secondary units in a closed hydraulic circuit, wherein the secondary units are normally arranged in parallel with respect to one another in terms of the circuit configuration. An internal combustion engine, for example a diesel engine of the mobile working machine, is coupled rotationally conjointly to the primary unit, and an output, for example an axle or a wheel of the mobile working machine, is coupled rotationally conjointly to the secondary unit. The mobile working machine thus has a traction drive which has a hydrostatic transmission.
The mobile working machine can also be braked using such hydrostatic transmissions. Here, in relation to traction operation, the power flows in the reverse direction from the output, via the secondary unit acting as a pump and via the primary unit acting as a motor, to the internal combustion engine, which is then driven in a passive cranking operating mode.
From the applicant, hydrostatic transmissions are known which have a speed control system with a two-position closed-loop controller. If the actual traveling speed of the mobile working machine in question reaches a fixed activation threshold—in particular a maximum admissible traveling speed—a high-power braking operation is initiated and is performed until the actual traveling speed falls below a fixed deactivation threshold. This is a discontinuous speed control system, and braking is performed with a fixed braking torque.
The documents DE 10 2014 211 393 A1 and US 2014/0372000 A1 each disclose a hydrostatic transmission with a speed control system which identifies an overshooting of the braking power of the internal combustion engine and thereupon automatically triggers a high-power braking operation. A part of the braking power is output to the internal combustion engine, whereas another part of the braking power is converted by means of a pressure-limiting valve of the high-pressure line in question into heat. It is disclosed in a non-specific manner that the delivery volume of the secondary unit is implemented from the braking demand.
DE 10 2014 206 123 A1 has disclosed a method for identifying overrun operation and for calculating the braking torque on the basis of the overrun operation. Here, a difference between the setpoint and the actual traveling speed of the mobile working machine in question is taken into consideration, and is converted, by multiplication by a constant, into a braking torque which can be controlled by a control unit by means of the adjustment devices and thus by means of the swept volumes of the two units.